Nginx Proxy server to App Platform app is getting a 403 Access Denied - Cloudflare error

I’m having an issue trying to proxy pass to my App Platform application from another server.

I have a .NET 6 app running in a Docker container on App Platform. This is running fine. I hit my API from my REST client using the domain given to my app by App Platform.

What I’m trying to do now is add an entry to my server running Nginx to proxy requests from my domain to the application on App Platform.

server {
    listen              443 ssl;
    server_name         ~^(?<subdomain>[\w-]+)\.mydomain\.com$ mydomain.com;

    ssl_certificate      /etc/letsencrypt/live/mydomain.com/fullchain.pem;
    ssl_certificate_key  /etc/letsencrypt/live/mydomain.com/privkey.pem;

    location /api {
        proxy_pass              https://my-app-platform-app.ondigitalocean.app;
        proxy_http_version      1.1;
    }
}

This works fine, my api receives the request but the Host header of the request is the app platform domain (https://my-app-platform-app.ondigitalocean.app) but want my domain from my proxy server (https://mydomain.com) as the Host header. So what I did was set the Host header with proxy_set_header in my Nginx config like below.

server {
    listen              443 ssl;
    server_name         ~^(?<subdomain>[\w-]+)\.mydomain\.com$ mydomain.com;

    ssl_certificate      /etc/letsencrypt/live/mydomain.com/fullchain.pem;
    ssl_certificate_key  /etc/letsencrypt/live/mydomain.com/privkey.pem;

    location /api {
        proxy_set_header        Host $host;
        proxy_pass              https://my-app-platform-app.ondigitalocean.app;
        proxy_http_version      1.1;
    }
}

Now when I try to access my API from mydomain.com/api I get a 403 Permission Denied - Cloudfare error

Has anyone encountered this issue, or know what I’m doing wrong with this?

Thank you.